1. Field of the Invention
This invention relates generally to igniters for igniting fuel-air mixtures in combustion chambers, and more specifically to the energy efficiency of corona igniters.
2. Related Art
An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen. The corona discharge ignition system includes a corona igniter with an electrode charged to a high radio frequency voltage potential. Like igniters of other types of ignition systems, the corona igniter includes an ignition coil with a plurality of windings surrounding a magnetic core and transmitting energy from a power source to the electrode. An example of an ignition coil of a corona igniter is shown in FIG. 4. The corona igniter receives the energy at a first voltage and transmits the energy to the electrode at a second voltage, typically 15 to 50 times higher than the first voltage. The electrode then creates a strong radio frequency electric field causing a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture.
The ignition coil of the corona igniter is designed to create, in conjunction with the firing end assembly, a resonant L-C system capable of producing a high voltage sine wave when fed with a signal of suitable voltage and frequency. During operation of the corona igniter, an electric current flows through the coil, causing a magnetic field to form around the coil. Ideally, magnetic flux lines would follow the magnetic core through the entire length of the coil, exit the ends of the magnetic core, and then return around the outside of the coil. In this ideal situation, all the magnetic flux would be linked with all the windings, and the magnetic flux density would be equal at all radial cross sections of the magnetic core. Further, the magnetic core would ideally be sized according to the desired electrical behavior and the material properties and therefore would provide low electrical and energy losses.
In reality, however, the magnetic flux density is much greater in the center of the magnetic core, as shown in FIG. 5A, wherein the darker regions correspond to higher magnetic flux densities. The corresponding magnetic flux lines are shown in FIG. 7. The high magnetic flux density in the center occurs because a significant amount of magnetic flux passes partially through the magnetic core and then loops back radially through the windings prior to reaching the ends of the magnetic core. The increased magnetic flux density in the center of the magnetic core pushes the magnetic material toward saturation and ultimately results in high heat and high energy losses.
The magnetic flux that exits the magnetic core prior to reaching the ends of the magnetic core has a negative effect on the current flow through the windings. Where the magnetic flux passes through the windings, adjacent the opposite ends of the magnetic core, the current density within the windings is locally increased, as shown in FIG. 6A, such that the current density over the cross section of the windings is unequal. The increased current density results in increased resistance and thus higher energy lost as heat. The current flowing through the negatively affected windings is lower in the center of the wire, and the current is forced to flow through a relatively small cross-sectional area, adjacent the outer surface of the wire, relative to the total the cross-sectional area of the affected wire. This effectively reduces the functional and operational cross section of the wire and gives a far higher resistance, resulting in high energy losses.